A cathode-ray tube, such as a color television picture tube, comprises an evacuated glass envelope which includes a funnel, having a substantially rectangular opening at one end and a tubular neck at the other end, and a faceplate panel that is sealed to the one end of the funnel by a frit seal. It is common practice to bevel the sharp inside edges of the sealing surfaces of the funnel and panel to prevent chipping or bruise checking which can cause either particles within the tube or a failure of the evacuated tube.
U.S Pat. No. 3,550,322, issued to Rajnik et al. on Dec. 29, 1970, discloses an edge beveler which permits power grinding of the inside and outside seal edges of the funnel and faceplate panel. The edge beveler includes a vertical rotatable shaft having a plate member at the upper end thereof which has a peripheral shape generally corresponding to that defined by the seal edge of the article to be beveled. The plate member is smaller than the inner border of the seal edge and is located within the article. A plurality of rollers support the seal edge of the article while it is rotated by the vertical shaft. A pair of motor driven grinding wheels contact, at an angle, the inner and outer borders of the sealing edge of the article.
A drawback of the above-described edge beveler is that the sides of the substantially rectangular funnel and panel have very large radii of curvature whereas the curvature of the corners is small and it is therefore difficult to obtain a uniform bevel around the entire periphery of the seal edge. Additionally, the diameter of the grinding wheels must be small enough to accommodate the corners of the funnel and panel so the grinding wheels must be replaced frequently. Also, the same area on each of the grinding wheels contacts the borders of the seal edge further accelerating wear of the grinding wheels.
A need therefore exists for a method and apparatus of beveling an edge which minimizes the drawbacks of the prior art.